1. Field of the Invention
The present invention relates to a capacitance position sensor for determining the distance between two opposed electrodes by measuring the capacitance between them, to a position controller equipped with the sensor, and particularly to a capacitance position sensor suitable for application to mirror control in a laser system or the like.
2. Description of the Prior Art
Lasing is achieved by amplifying a laser beam as it travels back and forth between multiple mirrors. The lasing state is maintained by precisely controlling the relative positions of the mirrors and the direction (orientation) of their faces. Conventional position sensors for this kind of mirror control include optical and resistive types. The optical type consists of numerous components (five to 10 mirrors and other optical components) and is therefore difficult to miniaturize and high in cost. The size of the world's smallest commercially available optical position sensor (produced by Omron Corporation of Japan) is 33×39×17 mm. Although sensors using slide resistors are inexpensive, they are inferior in positional regulation owing to the need for physical contact, backlash of the slide resistor and other drawbacks
As at least several to more than ten mirrors are required for lasing, there is a pressing need for an inexpensive, accurate and compact position sensor. In fact, however, no position sensor that can be given high marks on all three of these points has yet emerged.
One commercially available capacitance position sensor is the capacitance bridge type position sensor marketed by ADE Corporation of the United States (see Canadian Patent No. 961,927). This also has the disadvantage of high price (around $1,700). Many conventional capacitance position sensors detect the capacitance by measuring the amplitude or phase of attenuated signal through the capacitance of the opposed electrodes, and have a problem of amplitude and phase fluctuation due to noise. Accuracy is therefore inferior owing to the considerable effect of noise.
Japanese Unexamined Patent Publication No. 11-201708 teaches another type of position sensor that utilizes in-plane inductance. However, it is clear from the graph of FIG. 20 in this publication that this position sensor does not achieve very high accuracy. On the other hand, Japanese Unexamined Patent Publication No. 9-17840 teaches a capacitance position (displacement) detector that detects position on a rail in the manner of the frame and slider of a vernier caliper. But it has low detection accuracy that is fundamentally impossible to improve beyond a certain limit owing to the structure requiring contact between the frame and the slider.
An object of the present invention is therefore to provide a position sensor that is compact, accurate, noise resistant and simple in structure. Another object of the present invention is to provide a position controller equipped with the position sensor. Still another object of the present invention is to provide a capacitance position sensor suitable for position control of the numerous mirrors used in lasing and the like and also suitable for multiaxial position control.